The present invention relates to a projection headlamp of the type in which a light beam reflected by a substantially elliptical reflector is projected in the forward direction of the vehicle by a projection lens. More particularly, the invention relates to an automotive-type projection headlamp capable of selectively changing a beam distribution pattern from one pattern to another by tilting a shade.
Conventional headlamps of this same general type are disclosed in Published Unexamined Japanese Utility Model Application No. Sho. 63-41801 and Published Unexamined Japanese Patent Application No. Hei. 1-213901.
The headlamp disclosed in Japanese Utility Model Application No. Sho. 63-41801 is constructed as shown in FIG. 1. As shown in FIG. 1, a light source 3 is positioned at the first focal point F.sub.1 of a substantially elliptically shaped reflector 2, and a shade 4 is positioned at the second focal point F.sub.2. A projection lens; 5 is located in front of the shade 4. In the headlamp thus constructed, for selectively changing a main beam distribution pattern, the shade 4 is turned about a horizontal support shaft 7 to partially intercept the light beam directed toward a projection lens 5. Reference numeral 1 designates a lamp body, and reference numeral 6 indicates a front lens.
The headlamp of the above-mentioned Published Unexamined Japanese Patent Application No. Hei. 1-213901 employs a shade turning arrangement different from that of Utility Model Application No. Sho. 63-41801, but the headlamps are both based on the same concept that, to select the main beam distribution pattern or the subbeam distribution pattern, the shade is turned.
The above-described conventional headlamps are capable of selecting between only two patterns--a main beam distribution pattern (high beam) and a subbeam distribution pattern (low beam). In other words, in such headlamps it is not possible to select a beam distribution pattern such as a medium beam distribution pattern of an intermediate state between the main beam and subbeam distribution patterns.
For highway driving, since the speed of the car is generally higher, the beam distribution pattern of the headlamp, when set at or close to the main beam distribution pattern, provides better visual recognition for the driver, but causes more intense glare to drivers of oncoming vehicles. It would therefore be very convenient if a medium beam distribution pattern could be used so as to not cause glare until the two vehicles come near each other within a certain distance.
As mentioned above, conventional headlamps could not select a beam distribution pattern other than the main beam distribution pattern and the subbeam distribution pattern. In this respect, the market has desired the development of a headlamp with which more than these two patterns can be selected.
To satisfy this market need, a headlamp has been proposed in commonly assigned Japanese Utility Model Laid-Open Publication No. Hei. 5-33402 in which, as shown in attached FIG. 2, a horizontal support shaft 7 is offset from the center axis O of a tubular body 4 acting as a shade. The tubular body 8 or the shade is turned about the horizontal support shaft 7. As the shade turns, the ridge of the shade 8 is vertically moved, whereby the cut line of the light beam distribution pattern is varied in a continuous manner. However, this approach still has problems, as will now be explained.
The horizontal support shaft 7, which defines the center of rotation of the shade, is not coincident with the center of gravity of the tubular body. In other words, the center of shade rotation is not coincident with the center of gravity of the tubular body. Accordingly, the load on the motor acting as the drive source varies in accordance with the position of the tubular body as it turns. This load variation hinders smooth rotation of the shade, viz., smooth variation of the distribution pattern.
Further, vibration caused by the car engine is unavoidably transmitted through the horizontal support shaft 7 to the tubular body 4. In this case, since the horizontal support shaft 7 supporting the tubular body is not coincident with the center of gravity thereof, some types of vibration can give rise to a high torque acting on the horizontal support shaft 7.
Moreover, vertical movement of the vertex of the shade causes displacement of the shade vertex in the direction of the optical axis. As a result, the shade vertex is displaced from the meridional image surface f. The resultant cut line is unclear. In the figure, P.sub.1 indicates the vertex position of the shade (where the shade contributing to formation of the cut line in the distribution pattern is at the highest position) when the shade 8 is positioned as indicated by a solid line, P.sub.2 indicates the vertex position of the shade (where the shade is turned a preset angle counterclockwise) when the shade 8 is positioned as indicated by a phantom line.